The data sheet 10/18-0.22-DE from ABB discloses an electropneumatic position regulator which, in order to determine the position of an actuator, has a displacement sensor which is in the form of a rotary pick-up and scans an angle of rotation of up to 270°. A so-called fitting kit is used to fit the position regulator in such a manner that the position is determined directly in the case of rotatory actuators and is determined indirectly via a lifting movement which is converted into a rotational movement in the case of linear actuators or lifting drives.
The term “position regulator” used in this disclosure refers to a mechatronic system which controls the auxiliary energy of a pneumatic actuating drive according to one or more input signals in order to move the actuating member into a particular position. For operation, the digital pneumatic position regulator uses pressurized gas as auxiliary energy and electrical energy.
Known digital pneumatic position regulators include at least the core components described in more detail below. The chambers of a single-acting or double-acting pneumatic drive can be ventilated or vented in a targeted manner on the basis of one or more input signals using a pneumatic system. The movements and positions of the actuating member can be represented as one or more signals with the aid of a position feedback sensor system. Control electronics which have a microcontroller and receive one or more input signals can also be provided. The firmware in the control electronics processes the input signals and the signals from the position feedback sensor system to form output signals which can be used as input signals for the pneumatic system.
The firmware of the digital position regulator implements a function which analyzes the dynamic properties of the connected actuating drive. During start-up, the actuating range of the actuating member can be moved through once during an initialization process, and the initial value and final value of the actuating range are recorded.
Actuating drives can be subdivided into pivoting drives and lifting drives. In the case of a lifting drive, the linear movement of the output drive of the actuating drive is directly transmitted to a linearly operated actuating member. In contrast, in the case of a pivoting drive, the linear movement of the output drive of the actuating drive is converted into a rotational movement using suitable means.
In order to measure the position, it is known practice to use a potentiometer with a slider tap as the sensor. However, these potentiometers are not sufficiently robust with respect to vibration/shaking and the sliders wear out after a finite number of movements. Chemical influences may also affect the service life of slider potentiometers. This can result in failure of the displacement measurement and thus in failure of the device function. In addition, such potentiometers with a slider tap have a measurement range limited to, for example, approximately 270° and have hysteresis which cannot be ignored.
To increase the robustness of the sensor, so-called magnetoresistive sensors (called MR sensors for short below) are appropriate. A suitable variant of such an MR sensor is the magnetoresistor. A magnetoresistor changes the resistance when subjected to magnetic influence. A known, ready-made subassembly is designed like a potentiometer. It includes, in the interior, a connected magnetoresistor arrangement above which a permanent magnet is vertically arranged in a contactless manner with respect to the magnetoresistor.
The magnetoresistor is electrically contact-connected and the electrical connection is routed to the outside for use in an evaluation circuit. A shaft associated with the magnetoresistor potentiometer is rotatably mounted in such a manner that a rotatory movement can be recorded. The shaft is guided into the interior of the magnetoresistor potentiometer via a bearing. The permanent magnet is permanently mechanically connected to the shaft there. The shaft makes it possible to contactlessly rotate the permanent magnet via the magnetoresistor arrangement according to the rotational movement recorded from the outside and thus makes it possible to change the electrical resistance of the magnetoresistor arrangement.
However, above the angle of rotation of 360°, the transfer function of such a magnetoresistor arrangement has a characteristic which resembles a sine function. This is not suitable for measuring a position in the entire range of the characteristic curve. The vertices have a gradient which is too low for a position measurement or do not have a gradient at all, which could be metrologically recorded and evaluated using, for example, an A/D converter. Therefore, two ranges which are each per se approximately 120° . . . 140° are available to this sensor. The measurement ranges differ in terms of their gradient. A rising gradient of 0° . . . 180°, a falling gradient of 180° . . . 360°, in contrast to a conventional slider potentiometer which has a change in resistance (when rotated) which rises in a strictly monotonic manner and an undefined range in which the slider leaves the resistance track. After passing through the undefined range, the slider of the slider potentiometer taps the resistance track again at its origin.
In the case of known position regulators, this property of the slider potentiometer is used to determine methods of operation. Since the zero point position on the position sensor is achieved in known lifting drives by rotating to the left, the operating range is described using two position points, the point with the lower resistance being defined as 0% and the point with the higher resistance being defined as 100% of the operating range. It can thus be firmly assumed that the drive approaches its 0% position when rotated to the left and approaches the 100% position when rotated to the right. This is only possible because the position sensor has a characteristic which rises in a strictly monotonic manner, and a resistance value uniquely identifies a position.
Such a fixed association between the direction of movement and the effective direction of the drive is only possible in a magnetoresistor potentiometer when it can be ensured that either the range 0° . . . 180° or the range 180° . . . 360° is used as the sensor range. However, this restriction cannot be ensured for reasons of the application.
For example, in applications in which the usable sensor range of approximately +/−70° (140°) does not suffice, the measurement range is mechanically spread using a transmission ratio, for example a ratio of 2:1. Because the displacement pick-off shaft does not have any mechanical limitation and is rotated twice—that is to say through 720°—so that the sensor undergoes one revolution (360°), the effect occurs where the measurement range final value—for example the 0° point—is sometimes in the first measurement range of the sensor (0° . . . 180°) and sometimes in the second measurement range (180° . . . 360°). The gradient at the same measuring point is therefore sometimes rising and sometimes falling, which precludes an association between the direction of movement and the effective direction of the drive.